Modelos SERVQUAL

The Seram Trough extends WNW from approximately 50 km offshore

northwest of west Seram (Fig. 1.2) and swings SE offshore west of Onin Peninsula in the Kepala Burung area. On bathymetry (Fig. 4.3) the trough appears to be delineated by the depression represented by the 1000 m contour line and deepens as it approaches its western termination east o f Sulabesi. On the interpreted seismic profiles of Letouzey et al. (1983) the Seram Trough is clearly defined as a sea-bottom depression reaching to more than 4 s TWT (Fig. 4.4). The southern part o f the trough is characterised by intensely deformed sediments forming north-dipping thrust packets but to the north, a thick sequence o f relatively undisturbed sediments (about 2 s TWT thick) rests on acoustic basement at about 5 s TWT depth.

4.2.7 N orth Sula-Sorong Fault

Passing south o f the Obi Island (Fig. 1.2), the North Sula-Sorong Fault extends westwards and continues as far west as northeast Mangole before it transmutes into a zone o f thrusting, the Sula Thrust, (Letouzey et al. 1983, Silver et al. 1983). On bathymetry (Fig. 4.3) the North Sula-Sorong Fault and the Sula Thrust appear to

coincide with the 1000 m contour line which runs parallel to the north coast of Mangole and Taliabu and as far west as a major strike-slip fault, the Greyhound Strait Fault (Hamilton 1979) which separates Taliabu from the Banggai Islands. Silver et al. (1983) interpreted the Sula Thrust as a zone o f moderate to low-angle thrust faulting (Fig. 4.5), which brings the tectonic melange o f the Molucca Sea onto the Banggai-Sula

Microcontinental Platform (see Section 4.5.1).

The interpreted seismic profiles (Fig. 4.4 profiles 1, 2 and 3) showed that the North Sula-Sorong Fault is represented by a sea-bottom depression reaching to more than 2 s TWT (profiles 1 and 2). On profile 3 however, the fault zone is less distinct than on profiles 1 and 2. The depression is bounded on both sides by moderate to gently

sloping seafloor, which can be interpreted as representing a zone of transtensional faulting (Reading 1980, Park 1989).

4.2.8 South Sula-Sorong Fault

The South Sula-Sorong Fault separates from the North Sula-Sorong Fault immediately south o f Obi and appears to extend as far west as the NW-SE oriented Greyhound Strait Fault (Fig. 1.2) west of Taliabu. Defining the path of the fault is complicated by the presence o f the north-south oriented island o f Sulabesi. The fault may pass between Sulabesi and Mangole but Charlton (1995) suggests that it actually suffers a right lateral offset along a transfer fault which coincides with Sulabesi. This question is further considered in Chapter 5, which deals with the sidescan sonar data and in the discussion of the gravity modelling (Chapter 6). The fault continues across the Greyhound Fault as far west as the area southwest o f Peleng, as suggested by the

1000 m bathymetric contour line and the steep bathymetric slope (Fig. 4.3). It may therefore form the entire terrane boundary which separates the Banggai-Sula continental fragment from the North Banda Sea terrane (Fig. 4.1). Seismic profiles were not

available in this area and the interpretation was made chiefly on the bathymetric information. Gravity modelling in this region also is analysed in Chapter 6.

4.3 Oceanic Terranes

Possible sources o f oceanic terranes within the Sorong Fault Zone and the surrounding region include the Indian Ocean and the Philippine Sea oceanic crusts. However, neither of the two oceanic terranes which are recognised in the fault zone, the Molucca Sea and the North Banda Sea terranes, appear to have come from either of these oceans. The Molucca Sea represents the remnant o f a formerly much larger ocean now almost entirely subducted, whereas the origin o f the North Banda Sea terrane remains ambiguous.

4.3.1 Molucca Sea Collision Zone

The Molucca Sea Collision Zone is situated in a region of complex interaction between the Eurasian, Australian and the Philippine Sea plates and is composed largely of intensely deformed sedimentary rocks. Hamilton (1979) noted that the rocks which underlie the Molucca Sea are acoustically irresolvable. Katili (1975) recognised that subduction zones exist adjacent to the Sangihe and Halmahera arcs facing the Molucca Sea, and considered that the deformed trench fills may have been responsible for the irresolvability of seismic images in this region. Isostatic gravity anomalies reaching as low as -200 mGal (Vening Meinesz 1948) indicate either that there are anomalously low densities o f crustal or mantle material beneath the region, or considerable thickness of sedimentary rocks. More recently McCaffrey and Silver (1980) studied seismic refraction profiles in the region and found that an approximately 15 km thick low- velocity layer (collision complex) occurs beneath the 2 km water depth of the Molucca Sea. They suggested that this collision complex can account for the free-air anomaly, which reaches values as low as -250 mGal in the southern part of the region.

The Halmahera arc on the east and Sangihe arc on the west are both active and face towards the Molucca Sea. Two Benioff zones which dip away from the Molucca Sea are clearly defined by earthquake focal depth information in the region (Fig. 4.7). The east-dipping Benioff zone results in the active volcanic arc o f western Halmahera. The active Sangihe volcanic arc is the direct consequence of the west-dipping Benioff zone. The magmatic arcs of Halmahera and Sangihe are separated by approximately 250 km at the closest distance. The active volcanoes o f the Halmahera arc are situated immediately west o f the west coast o f Halmahera in the central part o f the island, and immediately onshore in the northern part o f the island. The active volcanoes o f the Sangihe arc are located as far south as the northern tip of the island o f Sulawesi. The north and south arms of Sulawesi are composed o f inactive arc volcanics. Nearly continuous volcanic activity has been inferred from about the Early Miocene up to Quaternary time (Sukamto 1975). The East Sangihe Thrust and the West Halmahera Thrust form the west and east boundaries of the Molucca Sea Collision Zone and are marked by troughs 3000 m deep bordering the arcs along the sides facing the Molucca

Sea. The East Sangihe Thrust separates the collision zone from the North Sulawesi arc- volcanic terrane, whereas the West Halmahera Thrust isolates the collision zone from the West Halmahera-Tamrau arc terrane. The two thrusts should be regarded as superficial features developed during collision and not as the surface traces of the subduction zones. The southern boundary of the collision zone is the Sula Thrust along which the Molucca Sea collision complex overrides the Banggai-Sula Microcontinental Platform.

The north-trending central region of the Molucca Sea Collision Zone is a broad bathymetric high, the Talaud-Mayu Ridge, on which the Talaud, Mayu, and Tifore islands emerge above sea level. Exposures of peridotite and gabbro were found on the various islands during the course o f the Sorong Fault Zone Project, indicating up- thrusting o f slices o f mantle material above sea level. Local gravity highs reaching to more than +100 mGal on Mayu and +200 mGal on Talaud (Sardjono 1992) marks the occurrence of high level slices of mantle material. A crustal and lithospheric gravity model o f the Molucca Sea collision zone proposed by McCaffrey and Silver (1980) is shown in Fig 4.8. Shallow earthquakes are concentrated beneath the crest of the ridge and indicate a predominance of thrust type focal mechanism (Fitch 1970).

4.3.2 N orth Banda Sea

The North Banda Sea terrane occupies a region in the north-western part o f the Banda Sea Basin (Fig. 4.1). It is bounded in the north by the South Sula-Sorong Fault which separates it from the Banggai-Sula Platform. A strand o f the Tolo Thrust forms the west and southwest boundaries and isolates the North Banda Sea terrane from the East Sulawesi Ophiolite province and the Central Sulawesi Metamorphic belt. The Banda Ridge (Sinta Ridge; Réhault et a l 1991) in the south (Fig. 1.2), forms the southern boundary and separates the North Banda Sea terrane from the South Banda Sea Basin. The eastern boundary o f the North Banda Sea terrane is largely formed by the island o f Buru, which is believed to be a detached portion o f the Australian continent (Pigram and Panggabean 1984).

Réhault et a/. (1991) recognised the North Banda Sea terrane as part of the Banda Sea oceanic crust. Bowin et al. (1980) suggested that the Banda Sea might be a trapped fragment of oceanic crust of Cretaceous-Eocene age originally part o f the Argo Abyssal Plain northwest o f the Australian continent. The oceanic crust of the North Banda Sea would therefore be expected to have characteristics similar to Indian Ocean crust. Based on the patterns o f marine magnetic lineations in the Banda, Sulawesi and Sulu basins, Lee and McCabe (1986) claimed that these basins were a continuous feature in the Cretaceous to Early Tertiary time and that the various islands which either arrived at their present position as a result of middle to late Tertiary tectonic

movements or emerged as a result of the Neogene subduction, have dissected and isolated the continuous ocean basin into its present configuration (Fig 4.1).

If the sliver kinematics o f the tectonic model proposed by Charlton (1986) are valid, only the southern and south-eastern parts o f the Banda Sea are underlain by Indian Ocean crust (Fig. 4.2). The northern and north-western parts of the Banda Sea could, therefore, be underlain by oceanic crust which originated from the Philippine Sea Plate. However, on the basis of geochemical analyses and radiometric age dating of rock samples (± 6 Ma, Late Miocene to Early Pliocene) dredged from the floors o f the North and South Banda Sea basins, Réhault et al. (1994) concluded that the two basins are o f similar nature. Furthermore, they hypothesised that the North and South Banda Sea resulted from the same process of back-arc opening during the Late Neogene. These basins are now separated by the NE-SW trending Banda Ridge, o f continental in character, from which samples o f Triassic platform carbonate were dredged (Réhault et al. 1994). The continental sliver Banda Ridge may have originated at the northern margin o f the Australian continent and have been translated by the left-lateral

movement o f the Sorong Fault into its position separating the North Banda Sea terrane from the South Banda Sea basin. If so, it possibly reached the present position before spreading commenced.

4.4 Arc Terranes of the Philippine Sea Plate

Arc terranes in the Sorong Fault Zone originated primarily from the interaction between the Philippine Sea Plate and the northern margin of the Australian Plate (Hall et a l 1987). Convergence at the plate boundaries resulted in the development of volcanic island arcs. These are recognised as the West Halmahera-Tamrau terrane, the East Halmahera-Waigeo terrane (Fig. 4.1) and the Arfak terrane; the latter is located in the extreme northern part o f Kepala Burung (Fig. 4.6).

4.4.1 W est H alm ahera-T am rau T errane

The West Halmahera-Tamrau terrane is defined here as occupying the region covering the Northwest and Southwest Arms of Halmahera and the recent volcanic islands immediately to the west of the west coast o f Halmahera (Fig. 4.1 and Fig. 4.9). It extends to the east to include Batanta, north Salawati and the northern part of the Kepala Burung. The West Halmahera thrust to the west forms the western boundary of the terrane, isolating it from the Molucca Sea terrane. The Molucca-Sorong Fault to the south represents the southern boundary, separating the West Halmahera-Tamrau terrane from the Obi province and terranes in the Kepala Burung region. On Halmahera, the eastern boundary of the West Halmahera-Tamrau terrane is a suture zone situated in the central area (Fig. 4.9). This is a zone o f strong deformation in which Neogene rocks have been locally and intensely deformed (Hall et a l 1987). The West Halmahera- Tamrau terrane extends northward to the eastern part o f the Philippine volcanic belt (H alU ra/. 1987).

The West Halmahera-Tamrau terrane consists mainly o f pre-Late Cretaceous island arc volcanic and volcaniclastic rocks (Hall et a l 1987). These rocks form the basement o f the province and are unconformably overlain by Late Miocene-Early Pliocene sedimentary rocks which record events in the transport o f Halmahera

westward along faults which may be related to the present day Sorong Fault Zone. The basement rocks consist o f calc-alkaline volcanic rocks, intrusive igneous rocks and

volcaniclastic units containing similar calc-alkaline debris. Lithologically, these rocks are andésites, andesitic breccias and conglomerates. The typical volcanic basement rock is represented by the Oha Volcanic Formation (Hakim and Hall 1991).

4.4.2 E ast Halmahera-W aigeo T errane

The East Halmahera-Waigeo terrane (Sukamto 1986, Hall et a l 1987) occupies

a region covering the Northeast and Southeast Arms o f Halmahera, Waigeo island and numerous small islands between these two including Gebe and Gag (Fig. 4.1). The central suture zone marks the western boundary o f the terrane, separating it from the West Halmahera-Tamrau terrane (Fig. 4.9). To the south, the East Halmahera-Waigeo terrane is bounded by the Molucca-Sorong Fault, isolating it from the Obi terrane. It is presumably separated from terranes in the Kepala Burung region by a fault south of Waigeo. The entire terrane forms a part of the Philippine Sea plate.

The East Halmahera-Waigeo terrane is composed o f ophiolitic basement complex consisting of a complete sequence, with the possible exception o f the sheeted dykes, o f ophiolite members ranging from ultramafic rocks, cumulates and

microgabbros, and volcanic rocks. Radiolarian cherts are common as float in areas of exposed ophiolites. Metamorphic rocks are also found, including foliated amphibole and minor blueschists. The plutonic igneous rocks include diorites and lesser granitic rocks (Hall et a l 1987). These rocks form the basement o f the Southeast and Northeast Arms of Halmahera and probably underlie the entire region between Halmahera,

Waigeo and North Obi (Charlton and Partoyo 1991, Hakim and Hall 1991). Peridotites include abundant serpentinized harzburgites and rare Iherzolites. The harzburgites record evidence o f a high degree of partial melting o f the mantle and are similar to those o f oceanic forearcs. The Iherzolites, in contrast, are less depleted than the harzburgites, and are compatible with a mantle residue after the extraction of mid- oceanic ridge basalts (Hall et a l 1987). Cumulates are common and consist of dunites, olivine clinopyroxenes, wehrlites and olivine gabbronorites. These indicate moderate to high degree partial melting of mantle materials. Chemical and petrological data show a

genetic relation between cumulates and harzburgites. Hornblende-rich diorites and trondhjemites which intrude the microgabbros have no genetic relation with the ophiolite pluton. Two phases of Late Cretaceous arc-related igneous activity were identified from "^°Ar/^^Ar dating of diorite hornblende. Volcanic rocks in the ophiolite complex include boninitic rocks and amygdaloidal calcalkaline basalts. These rocks have a composition similar to those o f ocean island volcanic rocks and seamounts (Hall etal. 1987).

The basement rocks o f Waigeo island (Fig. 4.10) consist of lithology similar to ophiolites found in East Halmahera and also are in a similar stratigraphie position and o f similar age (Charlton and Partoyo 1991). The ophiolites o f Waigeo include all members of the sequence from ultramafic rocks, through cumulates and microgabbros, to volcanic rocks. These rocks form the basement o f western and central Waigeo and possibly underlie the entire island. The upper contact between the basement and the younger sequence is always an unconformity or a fault. The basement rocks o f Waigeo are composed o f deformed and extensively serpentinized ultrabasic rocks including dunites and harzburgites with smaller quantities o f gabbros, dolerites and basalts. A large proportion o f the ultramafic rocks have cumulate textures, and represent the lower part o f a layered sequence. The western part of the island is composed predominantly of serpentinites and the north coast of the island shows massive exposure o f similar rocks. The age o f the ophiolitic rocks on Waigeo is not known. Massive and brecciated serpentinites are unconformably overlain by sandstone o f the Upper Eocene Lamlam Formation. The age of the basement complex is suggested to be a Jurassic or older (Hall e ta l 1987).